Abstract: A spring biased poppet valve has the exposed end of its poppet plunger engaging the underside of a lever, and the poppet and lever are biased upwardly so that a tit on the upperside of the lever engages a cam surface defined on a cam member loosely mounted on a generally horizontal rock shaft. The shaft can be moved through an angular range of displacements as part of a normal control system, which system receives fluid operating pressure through the open poppet valve. An adjustable stop screw mounted to the shaft is so oriented that the cam member is positively moved about the axis of the shaft when the shaft has reached a predetermined angular displacement, whereby further movement of the shaft permits the spring loaded poppet valve to close.